1. Field of the Invention
The present invention relates to a cordless telephone set including a cordless handset provided with various functions such as the transmitting and receiving functions necessary for telephone apparatuses, and a charger for charging a storage battery incorporated in the cordless handset.
2. Description of the Prior Art
For example, an extension telephone set comprises a main telephone apparatus connected to the telephone lines and one or a plurality of sub telephone apparatuses connected to the main telephone apparatus. In recent years, cordless telephone sets have rapidly been spreading in which the sub telephone apparatus is connected to the main telephone apparatus by wireless to make the sub telephone apparatus more convenient.
A conventional cordless telephone set used for this purpose comprises, as shown in FIG. 1, a cordless handset 31 serving as a telephone apparatus, and a stand-type charging apparatus 32 (hereinafter referred to as charging stand). Taking up the handset 31 from the charging stand 32, the user can make a telephone call at any place without any restriction by the cord. When the handset 31 is set in its position, that is, in a concave 33 of the charging stand 32 after the telephone call, the charging of a storage battery incorporated in the handset 31 is started.
The charging stand 32 normally has an indicator 34 constituted by a light emitting diode (LED) at a position such as an upper front portion which is easily viewed by the user. The indicator 34 is turned on when the handset 31 is correctly set in the charging stand 32, thereby notifying the user that the charging is being performed.
FIG. 2 shows an example of an electrical arrangement for charging the storage battery of the handset 31 in the conventional cordless telephone set provided with the above-described function. Contacts c1 and c2 to be connected to the electrodes of a storage battery 25 are provided at the bottom of the handset 31. Contacts d1 and d2 are provided at the bottom of the concave 33 in which the handset 31 is set. The indicator 34, a resistor 36 and a charging power supply 37 are provided for the charging stand 32.
In this conventional arrangement, when the handset 31 is set in the concave 33 of the charging stand 32, if the contacts c1 and d1 and the contacts c2 and d2 are correctly connected, a closed loop of from the power supply 37 through the storage battery 35, the resistor 36 and the indicator 34 to the power supply 37 is formed, so that the charging current of the storage battery 35 flows to the indicator 34 to turn it on. In this case, if a battery of a type where the current continues to flow after the battery is completely charged is used, the indicator 34 will continue to be on as far as the contacts c1 and d1 and the contacts c2 and d2 are connected.
FIG. 3 shows another conventional arrangement, in which a closed loop consisting of the same elements as the above-described arrangement except the indicator 34 is formed when the contacts c1 and d1 and the contacts c2 and d2 are connected, and a circuit 38 which detects a voltage change between the contacts d1 and d2 is further provided. The detecting circuit 38 detects that the voltage after the contacts c1 and d1 and the contacts c2 and d2 are electrically connected is lower than the voltage before the connection, that is, the supply voltage. The circuit 38 also drives a separately-provided indicator driving circuit 39 with the detection signal as a trigger.
In this arrangement, when the contacts c1 and d1 and the contacts c2 and d2 are electrically connected, the driving circuit 39 drives the indicator 34 to be continuously on, and when the contacts c1 and d1 and the contacts c2 and d2 are electrically disconnected, the driving circuit 39 is disabled, so that the indicator 34 is turned off.
In the above-described conventional arrangements, when the user sets the cordless handset 31 in the charging stand 32, the indicator 34 is turned on only when the contacts c1 and d1 and the contacts c2 and d2 are electrically connected, thereby notifying the user that the storage battery 35 is being charged.
In the above-described conventional arrangements, however, the indicator 34 is not turned on in the following two cases: when the handset 31 is not set in the charging stand 32; and when although the handset 31 is set in the charging stand 32, the contacts c1 and d1 and the contacts c2 and d2 are not electrically connected because they are not connected in good condition.
The latter case further includes the following two cases: when the contacts c1 and d1 and the contacts c2 and d2 are mechanically separated because the handset 31 is not set in a correct manner; and when although the handset 31 is correctly set and the contacts c1 and d1 and the contacts c2 and d2 are in contact with each other in a correct manner, the contacts are electrically disconnected since they are rusty.
Of the above cases where the indicator 34 is not turned on, the former case where the handset 31 is not set in the charging stand 32 corresponds to cases such as when the user is using the telephone and when the handset 31 is being cleaned. In these cases, it does not matter that the indicator 34 is turned off. However, the latter case is considered as a case where the user has not noticed that the indicator is off.
In the conventional arrangements, when the latter case occurs, the charging of the storage battery 35 is not performed although the handset 31 is set in the charging stand 32, and consequently, a telephone call may abruptly be interrupted the next time the user uses the telephone.
This problem may be solved if the user confirms the turning on of the indicator 34 when setting the handset 31 in the charging stand 32. However, various data have revealed the fact that most of the users do not confirm the turning on of the indicator 34. This is attributed to the fact that the light source of the LED used as the indicator 34 is so small that seeing if it is on is difficult in the bright sunlight.
To cope with this problem, the use of a large-light-quantity light emitting part as the indicator 34 and the use of a sound generating part such as a buzzer instead of the indicator are considered. However, in the above-described conventional arrangements, when the handset 31 is separated from the charging stand 32, the large-light-quantity indicator will continue to be on or the sound generating part will continue to generate a warning sound.